Heretofore, many different types of alignment and focusing systems have been employed with telescopic optical systems and they have met with reasonable success. However, larger space telescopes viewing extended objects such as the earth or a galaxy require new and improved means for measuring and correcting the optical aberrations of the telescopic system, and, when light weight mirrors are employed, active control of the mirror configuration is also needed. In the case of a segmented mirror optical system, the system of the invention is particularly suitable for sensing misalignment of the individual mirror segments without development of sensing instruments at the mirror center of curvature or at the secondary mirror, thereby eliminating the large structure required to reach the center of curvature or obscuring the aperture.
One of the features of the present invention is that it is operable when viewing an unbounded random scene, whereas previous instruments of this type required a point source; such as a star for example or a bounded scene, such as a planet, for successful operation.